Process for obtaining ingots with a small amount of pipe and of segregation



3,024,508 PROCESS FOR OBTAHNING INGOTS WITH A SMALL AMQUNT F PIPE AND OF SEG- REGATION Andr Gretfe, Annecy, France, assignor to Societe dEiectrp-himie dElectro-Metaliurgie et des Acieries Electrlques dUgine, Paris, France, a corporation of France N0 Drawing. Filed July 2, 1956, Ser. No. 595,084 (Claims priority, application France July 22, 1955 6 Claims. (Cl. 22-216) This invention relates to a process for casting ingots and, more particularly, to a process for casting metal into an ingot mold containing slag where pouring of the metal is terminated at a point in the mold so that a layer of slag remains on top of the metal.

In a copending application Serial No. 595,318, filed July 2, 1956, now abandoned, of which Messrs Andr Gretfe and Henri Jolivet are coinventors, there is a disclosure of a process of casting metal ingots wherein the metal is poured into a mold substantially or completely filled with slag. According to this copending application, the cast metal forms with the slag a metal-slag emulsion in the mold and simultaneously displaces from the mold, slag which then overflows into a slag ladle. The metal-slag emulsion subsequently separates into a body of metal free from particles of slag and a body of slag free from particles of metal. The casting of metal is preferably terminated at a point below the top of the mold so that there remains above the body of metal a layer of slag.

Heretofore, many metal pouring techniques and methods have been employed for reducing the amount of pipe and of segregation in metal ingots. Ability to reduce the amount of pipe in an ingot increases the yield of products rolled or fabricated therefrom by reducing the amount of metal which must be cropped off of the ingot or off the products rolled or fabricated therefrom. Pipe or segregation in the ingot necessitates cropping off a part thereof to obtain sound metal products, and it is essential that substantially all of the metal containing any pipe or segregation be cropped off, otherwise rejections of finished or semi-finished articles are likely to result. Generally, the croppings are scrapped and recharged into a metal melting furnace although in some instances, it is possible to divert parts of croppings to orders which require less strict specifications.

Ability to reduce the amount of segregation in metal ingots not only increases yield of usable metal, but also effects greater uniformity in both chemical and physical properties of the metal by reducing the amount of alloy concentration in a relatively small part of the ingot, thus avoiding parts of the ingot which are somewhat poor in alloy content.

Among better known processes heretofore used to reduce pipe or segregation in steel ingots are those which keep molten the upper part of the metal cast into a mold or into a hot top surmounting a mold by additions thereto of substances having exothermic properties and/or by subjecting the metal in the upper part of the mold to an electric are or to a flame. However, these processes have been unsatisfactory because of their low thermal eificiency and because of their likelihood of altering the composition of the upper part of the ingot by addition of elements, many of which affect the chemical and physical properties and the micrographic cleanliness of the metal. Other types of additions heretofore used have generated reactions with some of the components of the metal, thereby altering the chemical composition thereof.

I have developed a process for producing ingots which reduces the amount of pipe and/or segregation in the metal to a minimum and without altering the chemical composition of the upper part of the ingot. Specifically, my process comprises pouring metal into an ingot mold containing an overheated liquid slag and stopping the pouring of the metal into the mold at a point below the top thereof so that there is a layer of overheated liquid slag on top of the metal. Upon completion of the pouring of the metal, I maintain the layer of slag at a temperature above the solidification temperature of the metal until the metal has substantially solidified by introducing heat into the layer of slag.

In carrying out my process, I pour a quantity of overheated, liquid slag into an ingot mold, preferably an amount of slag sutficient to substantially fill or to substantially completely fill the mold. Then the metal to be cast is poured into the mold and simultaneously it displaces therefrom some of the slag. Before the mold or hot top surmounting the mold is completely filled with metal, I terminate the pouring of the metal so that a layer of slag remains on top of the metal in the mold. This layer of slag has sufficient thickness to limit loss of heat from the metal up through the layer of slag and thereby, prevent a premature solidification of the upper part of the ingot. Preferably, the layer of slag is about 10 to 20 cm. thick.

The slag which constitutes the layer above the metal must. have a composition such that it is liquid at the solidification temperature of the metal and such that it does not introduce elements into the metal which would alter the chemical composition thereof in a prejudicial way. Preferably, the heat conductivity of the slag should be low so that it functions as a good insulator with respect to the heat in the metal and so that the heat introduced into the slag is not dissipated at an excessively rapid rate to the surrounding air. I have advantageously used slags such as those described in the foregoing copending US. application Serial No. 595,318, filed July 2, 1956.

It is highly important that the temperature of the slag layer above the metal be maintained at a point above the temperature of solidification of the metal at least until the metal has substantially and preferably substantially completely solidified. To insure that the layer of liquid slag remains at a temperature above the solidification point of the metal, I introduce heat into the layer by adding an eXothermic substance such as one composed of granulated aluminum or silicon and of iron and manganese oxides which supply oxygen necessary for an exothermic reaction in the slag. Additions of exothermic substances are made at regular intervals during the solidification of the metal. Some of the exothermic substances which are satisfactory have the following compositions:

(a) Aluminum33.33%, ferric oxide and/or magnetic iron oxide-33.34%, manganese dioxide-33.33%

(b) Aluminum-33.33%, ferric oxide and/or magnetic iron oxide-66.67%

(c) Silicon28%, aluminum16%, ferric oxide and/or magnetic iron oxide-28%, manganese dioxide 28% There are, in addition to adding exothermic substances to the slag layer, other methods for maintaining the layer of slag at a desired temperature by introducing heat One example which demonstrates the advantages enjoyed by practicing my invention is set forth hereinafter.

A 1000 kg. reference ingot of 18% chromium, 8% nickel, low carbon, stainless steel is produced by casting the steel into a mold which has no slag therein. Following rolling of the ingot, 120 kg. of metal was cropped off and scrapped because of pipe and segregation.

Into a second mold of the same size and type as the one used for the reference ingot, 1000 kg. of the same metal was cast under slag in accordance with the process described in the US. application Serial No. 595,318, filed July 2, 1956, briefly discussed herein and the casting of the metal was terminated when the slag layer above the metal was about cm. in depth. The composition of the slag both throughout the cooling of the metal and its solidification was as follows:

SiOg, CaO, A1203, MgO, N820, TiOz, percent percent percent percent percent percent By introducing 3.3 kg. of an exothermic mixture at the rate of 300 g. every 3 minutes, the slag layer remained liquid for 33 minutes after completion of the metal pouring.

Following rolling of the ingot poured under slag, only 25 kg. of the metal had to be cropped off because of pipe and/ or segregation. Thus, practice of my invention increases yield from ingots by about 10% based upon a metal saving of 95 kg. in a 1000 kg. ingot compared to the 120 kg. loss experienced with the reference ingot.

My process has important advantages, one of which is a material reduction in the amount of pipe and/or segregation in cast ingots and in some instances, elimination of pipe and/or segregation, thereby providing a marked increase in yield from an ingot. A second advantage is improvement of micrographic cleanliness in the upper part of an ingot made possible by maintaining the upper part of the metal molten for a time after completion of casting, thereby providing non-metallic inclusions which tend to rise up through the metal with a better opportunity for escaping from the metal into the slag.

While certain preferred embodiments of my invention have been described, it Will be understood that it may be otherwise embodied within the scope of the appended claims.

I claim:

1. A process for obtaining steel ingots having good micrographic cleanliness and small amounts of pipe and of segregation, comprising pouring the metal into an ingot mold containing an overheated liquid slag, stopping the pouring of the metal into the mold at a point below the top of the mold so that there is a layer of overheated liquid slag on top of the metal, and thereafter adding heat to said layer to maintain the said layer of slag at a temperature above the solidification temperature of the metal until the metal has substantially solidified.

2. A process for obtaining steel ingots having good micrographic cleanliness and small amounts of pipe and of segregation, comprising pouring the metal into an ingot mold containing an overheated liquid slag, stopping the pouring of the metal into the mold at a point below the top of the mold so that there is a layer of overheated liquid slag on top of the metal, and thereafter maintaining said layer of slag at a temperature above the solidification temperature of the metal until the metal has substantially solidified by subjecting said layer of slag to a source of heat.

3. A process for obtaining steel ingots having good micrographic cleanliness and small amounts of pipe and of segregation, comprising pouring the metal into an ingot mold containing an overheated liquid slag, stopping the pouring of the metal into the mold at a point below the top of the mold so that there is a layer of overheated liquid slag on top of the metal, and thereafter adding an exothermic substance to said layer to maintain the said layer of slag at a temperature above the solidification temperature of the metal until the metal has substantially solidified.

4. A process for obtaining steel ingots having good micrographic cleanliness and small amounts of pipe and of segregation, comprising pouring the metal into an ingot mold containing an overheated liquid slag, stopping the pouring of the metal into the mold at a point below the top of the mold so that there is a layer of overheated liquid slag on top of the metal, and thereafter maintaining said layer of slag at a temperature above the solidification temperature of the metal until the metal has substantially solidified by subjecting said layer of slag to an electric are.

5. A process for obtaining steel ingots having good micrographic cleanliness and small amounts of pipe and of segregation, comprising pouring the metal into an ingot mold containing an overheated liquid slag, stopping the pouring of the metal into the mold at a point below the top of the mold so that there is a layer of overheated liquid slag on top of the metal, and thereafter maintaining said layer of slag at a temperature above the solidification temperature of the metal until the metal has substantially solidified by subjecting said layer of slag to an electric current.

6. A process for obtaining steel ingots having good micrographic cleanliness and a small amount of pipe and of segregation, comprising forming a metal-slag emulsion which separates into a body of metal free from particles of slag and into a body of slag free from particles of metal by pouring metal into an ingot mold containing an overheated liquid slag, stopping the pouring of the metal into the mold at a point below the top of the mold so that there is a layer of overheated liquid slag on top of the metal, and thereafter adding heat to said layer to maintain said layer of slag at a temperature above the solidification temperature of the metal until the metal has substantially solidified.

References Cited in the file of this patent UNITED STATES PATENTS 944,371 Monnot Dec. 28, 1909 1,074,251 Connell Sept. 30, 1913 1,160,169 Hering Nov. 16, 1915 1,310,072 Hadfield July 15, 1919 2,426,849 Udy Sept. 2, 1947 2,493,394 Dunn et al Jan. 3, 1950 2,567,525 Morris Sept. 11, 1951 2,631,344 Kennedy Mar. 17, 1953 2,694,023 Hopkins Nov. 9, 1954 FOREIGN PATENTS 1,091,963 France Nov. 3, 1954 

